The clinical, social and financial burden of Autism Spectrum Disorders (ASD) is staggering. They are the most prevalent of the developmental disorders and their incidence is rising. However, the ASD phenotype variability is large, and ASD symptoms can manifest over a range of ages and to different degrees. In part for these reasons, the ASD clinical diagnosis is challenging and often is not made until 3-5 years of age. Thus, there remains an unmet need for a valid and reliable endophenotype which would facilitate ASD diagnosis early in life, enable efficient study of ASD risk factors, and eventually serve as a useful biomarker to inform the development of effective therapies and assess treatment response in future clinical trials. The overarching goal of this proposal is to explore te utility of transcranial magnetic stimulation (TMS) measures of brain plasticity as a novel neurophysiologic endophenotype in high- and low-functioning adults and children with ASD. Our work to date demonstrates the potential utility of these measures in higher-functioning adults with ASD, and pilot data support the feasibility and safety of applying the same measures to children and lower functioning individuals in whom the value of such an endophenotype would be particularly high. We thus propose to apply single-pulse TMS to evaluate the modulation in corticospinal reactivity induced by a specific repetitive TMS protocol known as theta burst stimulation (TBS). The comparison of the motor responses induced by single-pulse TMS before and following TBS is a unique noninvasive measure of brain plasticity in humans, and we have found that it shows a reliable abnormality in high-functioning adult individuals with ASD. Our hypothesis is that the alteration of TBS-induced modulation of TMS responses is a common neuropathophysiologic trait that is reliably linked to the ASD phenotype, and that will not be limited to high functioning adults but be also valid in children and low-functioning individuals. W thus anticipate that data from the proposed studies will address an important need for a rapid, noninvasive, reliable and safe endophenotype available to patients with ASD across ages and level of function.